This is a circuit for correcting for fluctuations in laser beam power as the raster output scanner travels from one end of its scan to the other and specifically comprises a memory for storing and compensating for variations for a plurality of segments of said scan.
Laser xerographic printing systems in color, high-performance service are very sensitive to the amount of laser drum illumination when printing halftoned images. The effect of too much illumination is to make the diameter of the halftoned dots oversized, resulting in color shades too light and mismatched with the original. Too little illumination causes unwanted background to be developed in the white areas. The causes for this problem are various.
Some examples are:
(1) Side-of-drum to side-of-drum and drum-to-drum xerographic sensitivity variations.
(2) Optical scanner side-to-side efficiency changes. The changes result from design, fabrication and alignment abnormalities as well as day-to-day minute variations believed caused by such things as thermal gradients, mechanical straining of the optical bed, vibration and noise from machine mechanisms, etc.
(3) Laser radiation power output level. The useful life of a laser tube is usually determined by the length of time that a laser can furnish sufficient output light. For example, a typical laser may typically start at output levels at 18 to 22 milliwatts and are usable down to about the 5 to 8 milliwatt level.